• Arthropod communities and passerine diet: effects of shrub expansion in Western Alaska

      McDermott, Molly Tankersley; Doak, Pat; Breed, Greg; Handel, Colleen; Mulder, Christa (2017-08)
      Across the Arctic, taller woody shrubs, particularly willow (Salix spp.), birch (Betula spp.), and alder (Alnus spp.), have been expanding rapidly onto tundra. Changes in vegetation structure can alter the physical habitat structure, thermal environment, and food available to arthropods, which play an important role in the structure and functioning of Arctic ecosystems. Not only do they provide key ecosystem services such as pollination and nutrient cycling, they are an essential food source for migratory birds. In this study I examined the relationships between the abundance, diversity, and community composition of arthropods and the height and cover of several shrub species across a tundra-shrub gradient in northwestern Alaska. To characterize nestling diet of common passerines that occupy this gradient, I used next-generation sequencing of fecal matter. Willow cover was strongly and consistently associated with abundance and biomass of arthropods and significant shifts in arthropod community composition and diversity. Key nestling prey items were positively associated with both willow and ericaceous shrubs. Diet composition varied significantly among bird species and spatially within species, however, I found that temporal variability in prey abundance did not have a strong relationship to the probability of consumption. I predict that the wide temporal window of prey availability and high diet diversity may protect these birds against negative impacts from climate-driven shifts in prey phenology and abundance. Taken together, my results suggest that shrub expansion could result in a significant shift in Arctic food-web structure and an increase in food availability for insectivores, although future ecosystem change in the Arctic is likely to be heterogeneous as shrub types are expanding at different rates and in different places across the Arctic.
    • Assessing seasonal trends in harbor seal (Phoca vitulina) diet using stable isotope analysis along the whisker

      Smith, Justin Arthur; O'Brien, Diane; Horstmann, Larissa; Breed, Greg; Karpovich, Shawna (2017-12)
      Harbor seals (Phoca vitulina) that use tidewater glaciers in the summers to molt, pup, and breed have declined in recent years, which could be attributed to reduced prey availability caused by regime shifts in the coastal marine environment. Recording seasonal dietary trends of harbor seals in different years could improve our ability to test if regime shifts caused these declines. However, such analysis has not been possible, because wild harbor seals are difficult to recapture. Stable isotope analysis of serial sections of growing whiskers (mystacial vibrissae) can be used as a tool to assess diet over different seasons, but uncertainty about whisker growth status and shed dates have prevented accurate estimates of stable isotope deposition date in the past. In Chapter 1, I characterized harbor seal whisker morphology to improve estimates of stable isotope deposition date. First, I measured 567 whiskers collected from wild harbor seals in the Gulf of Alaska from 2003 to 2012. Measurements included the length of a smooth root section (SRS), the length of the bumpy section, and the distance between each bump (inter-bump length; IBL). I found that the SRS was longer for spring-collected whiskers than fall-collected whiskers and matched the length of fully-grown, shed whiskers. These results suggest that the SRS can be used to differentiate whisker shed and growth status, and can be used to determine the sequence of whisker shedding by cohort in summer-captured seals. I also found that the mean IBL was correlated with whisker length and provides a proxy for whisker growth rate. I compared stable carbon isotope ratios along the three longest whiskers from 10 harbor seals and found that intra-individual patterns of whisker stable carbon isotope ratios became more synchronous when expressed by deposition date rather than by position along the whisker. In Chapter 1, I proposed a method to improve deposition date estimates by applying individually adjusted growth rates and better estimates of shed date to wild harbor seal whiskers. In Chapter 2, I analyzed stable isotope ratios from serial sections of whiskers of 32 harbor seals from a population that uses tidewater glacial habitats in southeast Alaska. I used a mixed-effects repeated-measures model to determine the characteristics that influence stable isotope ratios over time. Mean stable carbon and nitrogen isotope ratios differed significantly among size classes (p < 0.005), with no effect of sex. Seals were then grouped by size to describe isotopic differences between different demographic groups using Standard Ellipse Corrected Area (SEAc). Larger seals (>1.4 m) exhibited a broader isotopic niche in the fall, winter, and spring relative to smaller seals (< 1.4 m), but had a similar niche width in the summer. These results suggest that seals using tidewater glacial habitat share common prey base in the summer, while larger seals diversify their diets throughout the rest of the year. Overall, the results of this thesis suggest whisker morphometric characteristics can be used to improve the ability to make longitudinal inferences using serial sections of the whiskers, which reveal differences in prey utilization by size class in harbor seals that merit further study.
    • Effects of diet quality and quantity on caribou and reindeer (Rangifer tarandus)

      Thompson, Daniel Paul; Barboza, Perry S.; Parker, Katherine L.; Kielland, Knut; Hundertmark, Kris J. (2013-05)
      Caribou and reindeer (Rangifer tarandus) encounter natural and anthropogenic disturbances across the landscape. In late winter, Rangifer encounter acute food from disturbances such as icing events. Furthermore, as shrubs expand into the Arctic tundra, the proportion of low quality browse may increase in the summer diet of Rangifer. This study evaluated how Rangifer tolerate 1) fluctuations in food quantity in late winter and 2) changes in forage quality over the summer. Rangifer can compensate for food shortages by increasing intake after restriction, which would allow animals to restore body mass quickly during migration. High body fat reserves increase the tolerance of food shortages. During the summer, Rangifer can consume exclusively browse to meet daily energy requirements; however, low nitrogen supply and high toxin load would require the use of alternative forages to supplement nitrogen and reduce toxins.
    • Resource partitioning between breeding common (Uria aalge) and thick-billed murres (Uria lomvia)

      Barger, Christopher Paul; Kitaysky, Alexander; O'Brien, Diane; Takebayashi, Naoki (2013-05)
      In seabirds, food availability is a driver of ecological and evolutionary processes. Here we examine how changes in food availability and energy demands affect both inter-specific resource partitioning and the genetic diversity within a species. We examined the effects of interannual fluctuations in food availability and predictable seasonal increases in energy demands on prey partitioning between breeding common (Uric aalge) and thick-billed (U. lomvia) murres. We observed strong spatial, temporal and dietary differences in the use of prey resources between the species. We found that partitioning increased as food availability declined and as energy demands increased during chick-rearing. We conclude that murres can buffer negative effects of warming and increased energy demands by reducing inter-specific competition for limited food resources. We also investigated the effects of contrasting foraging conditions and population trajectories on the genetic structure of common murres. We found that birds breeding on an increasing food-rich colony had higher genetic diversity than conspecifics breeding on a declining food-poor colony. This may be indicative of changes in a relative strength of purifying selection operating on increasing versus declining colonies. We conclude that foraging conditions might be driving the pattern of the genetic diversity in the Pacific common murre population.
    • The snowshoe hare filter to spruce establishment in boreal Alaska

      Olnes, Justin; Kielland, Knut; Ruess, Roger; Juday, Glenn; Genet, Helene; Mann, Daniel (2018-05)
      Interior Alaska is a heterogeneous landscape within the circumpolar boreal forest and is largely composed of black and white spruce (Picea mariana and P. glauca). Improving our understanding of the factors affecting patterns in spruce regeneration is particularly important because these factors ultimately contribute to shaping the boreal forest vegetation mosaic. Herbivory by snowshoe hares (Lepus americanus) is one factor that likely drives patterns in spruce establishment. The interaction between spruce and snowshoe hares provides an opportunity to study how plant-herbivore interactions can affect succession, vegetation community composition, and consequently, how herbivory influences landscape heterogeneity. I explored how herbivory by snowshoe hares alters the survival and growth of spruce seedlings across Interior Alaska's boreal forest. I hypothesized that the survival and growth rate of regenerating spruce is significantly reduced by snowshoe hare herbivory and that snowshoe hare herbivory influences the pattern of spruce establishment across time and space. To address this hypothesis, I conducted research in three distinct vegetation communities across the region: productive lowland floodplains (Chapters 1 and 2), treeline (Chapters 3 and 4), and recently burned stands of black spruce (Chapter 5). Together these five chapters reveal that snowshoe hares affect spruce establishment across much of boreal Alaska. Where and when hares are abundant, spruce can be heavily browsed, resulting in suppressed seedling growth and increased seedling mortality. The results of these studies also reveal a consistent and predictable pattern in which this plant-herbivore interaction takes place. The snowshoe hare filter acts as a 'spatially aggregating force' to spruce establishment, where the potential for optimal regeneration is highest during periods of low hare abundance and where hares are absent from the landscape.